Where is the best place to farm up Gems? I have been doing NM baal runs and this seems pretty good. My Windy Dru can just walk through and pick up freely. Never seemed like Hell Pits was too good for gems..
Nightmare cows have always given me a good amount of gems. Not to mention there is a higher likelyhood of more gems in the least amount of time considering the cow level isn't HUGE and the cows are tightly packed in heards for "farming".
Cows are probably the best for Gems, as stated above. Still, I'd take the liberty to recommend another, "fast" place: NM Eldritch/Shenk. It's a good place to lvl your mid lvl merc and while doing that myself, I noticed I get ALOT of flawless gems. It's probably the fastest place to get to too.
Another gem run worth considering: NM countess runs. By the time you get to the bottom of the tower cellar levels, if you kill along the way, you'll do quite nicely for yourself... and then you get runes at the end, as well. Quick, safe, and gets the job done.